The regulating mechansim involved in c-sis expression is being studied utilizing the technique of somatic cell hybridization. The strategy employed is to fuse cells negative for c-sis expression with those actively transcribing this gene. We postulate the existence of a trans-acting factor that will turn on a previously silent c-sis gene. Specifically, fusions between human glioblastoma or carcinoma cell lines expressed c-sis, while transcript, as determined by Northern blotting and hybridization under relaxed conditions using v-sis and human c-sis probes. Since the size of the mouse transcript was not known, it was necessary to find tissues or cell lines which expressed this gene. To do this, RNAs of several mouse cell lines, as well as embryos and placentas were screened. The transcript was detected in mouse placenta RNA and its size was found to be very similar to that of the human c-sis transcript. These results were make if difficult to distinguish between RNAs in the somatic cell hybrids. To overcome this problem, efforts are now underway to clone the mouse c-sis gene. Specific probes from this gene will be used to detect the mouse transcript in the cell hybrids using the Sl nuclease technique. This should distinguish the mouse c-sis RNA from the human counterpart. If positive results are obtained, attempts will be made to isolate the gene coding for a putative regulating factor utilizing molecular cloning and an appropriate selection system.